It is well-known that the quality and size uniformity of the wood chip input are important factors in the paper-pulping process. Further, it has been established that the thickness dimension of the wood chips is a key parameter in achieving the desired chip size uniformity. Although the optimum chip thickness in a particular situation will depend upon the particular wood fiber used and the particular type of pulping process employed, the generally desired range of chip thickness is between 6 and 10 millimeters.
Throughout the history of the manufacture of paper pulp, high operational efficiency and quality of the resulting pulp product have been industry goals. The equipment involved in the pulping process, such as the chip screening apparatus for the chip inflow, is typically characterized by high mechanical tolerance and precision, which must be maintained during the operating life of the equipment in order to achieve the desired results on a sustained basis. Accordingly, such equipment is typically very expensive and time-consuming to alter in any significant way after installation, even though it may be desirable to change the particular thickness dimension of a portion of, or all of, the chip screen apparatus in order to optimize results in a particular situation. This is a disadvantage of existing systems.
Another significant factor involving the quality and efficiency of the pulping process concerns the undersized chips, which includes both pin chips and "fines". It is an important goal that as few undersize chips in the chip inflow as possible be provided to the pulp digester. Further, it is important that as few new undersize chips as possible be created during the separation of the chip inflow into the different fractions. For instance, additional undersize chips are typically created when the over-thick chips are routed to the slicer, which reduces the size of the chips. Those undersize chips will ordinarily proceed to the digester.
With respect to the prior art, early examples of chip-screening systems are discussed in an article by E. Christensen in the May, 1976 TAPPI Journal, Volume 59, No. 5. One system disclosed in that article concerns chip thickness separation by means of a disk screen, wherein the rejected over-thick chips are removed from the chip inflow first and then directed toward a size-reduction apparatus such as a slicer. The remaining material, which includes both chips within the acceptable range (accepts) as well as undersize chips, is then generally sent to a second screening station, such as a gyratory screen, for fines removal. This is in fact one of the most commonly used systems in the pulping industry today.
An advance over that system is described in U.S. Pat. No. 4,376,042 to Brown. That patent discloses the use of a conventional gyratory screen with a modified top deck (screen), the function of which is to divide the inflow of chips into a first fraction of acceptable chips and a second fraction which includes both over-size and over-thick chips as well as some acceptable chips. The second fraction is then directed to a disk (thickness) screen which separates the acceptable chips from the over-thick chips, which are then sent to a conventional size-reduction apparatus. This particular arrangement of the two basic types of chip-screening devices (an initial sizing screen, such as a conventional gyratory screen, followed by a thickness screen, such as a disk screen) has proven to be significantly more effective and efficient than either the gyratory screen or the disk screen alone, and also more effective than a combination of gyratory screen and disk screen, with the disk screen being positioned first.
Although there previously has been investigation as to the optimum relative configuration and arrangement between a conventional gyratory screen and a conventional disk screen, there has been little, if any, investigation as to the optimum configuration and arrangement of the individual screening elements of a gyratory screen, including the relationship of such individual screening elements to a disk screen.